In my prior U.S. Pat. No. 3,804,988 I disclose a station carrier telephone system in which single sideband (SSB) amplitude modulation (AM) transmission is employed with the carrier being inserted in the transmitted signal to serve as a demodulation reference at the receiving end. In each channel, the subscriber carrier is an integral submultiple of the central office carrier, permitting the subscriber carrier to be derived from the central office carrier by frequency division and eliminating the need for a separate subscriber carrier source. A specific embodiment disclosed in that patent provides for a twelve channel system utilizing various 4 KHz frequency bands between 8 KHz and 136 KHz. It is desirable to provide additional channels for such a system is possible. However, in order to satisfy the requirement of a sub-multiple relationship between the central office and subscriber carrier frequencies, it becomes necessary to extend the frequency spectrum employed up toward and, in some cases, beyond 200 KHz. These higher carrier frequencies fall outside the range of the various frequency standards promulgated by the U.S. Rural Electrification Administration and other such agencies for station carrier equipment.
It is therefore an object of the present invention to provide both a method and apparatus for permitting optimal utilization on an available frequency spectrum for a station carrier telephone system.
It is a more specific object of the present invention to provide a single sideband station carrier telephone system in which there is no requirement for stable oscillators at subscriber circuits and which optimizes the utilization of the available frequency spectrum.
It is a further object of the present invention to provide a single sideband station carrier telephone system of the type described in which the subscriber and central office transmission bandwidths are the same size, thereby permitting the use of frogging repeaters along the transmission line.